In the prior art it has been known that it is advantageous in certain circumstances to have more than one power supply connected to a circuit in the event that one of the power supplies fails. In the past the power supplies have been placed in parallel, their connection to the electrical load being controlled by relays which may choose one of the supplies. In the event that the connected power supply fails, a control signal would activate the relays such that the power supply is disconnected from the circuit and another power supply would be connected to the circuit. One problem inherent in this type of design is the slow speed with which the contacts of the relays change position. This leads to a temporary lack of power at the load, which is undesirable. However, the use of relays does provide for low contact resistance between the power supply and the load, and for high isolation when the power supply is not connected to the load.
One might think that a static solid state switching device such as a Metal Oxide Semiconductor Field Effect Transistor (MOSFET) could be used under these circumstances. It is well known that MOSFETs switch quickly relative to relays.
MOSFETs have been tried for this application but have previously been found to be unsatisfactory. The reason for this is that during the construction of the MOSFET a diode is built into the MOSFET for protection reasons. When the MOSFET is installed in its normal fashion in a circuit, the diode is back-biased and consequently turned off and is not a part of the functioning circuit. In an application such as isolating a load from its power supply, when installed normally (i.e. drain connected to supply, and source connected to load), the diode may become forward biased and conducting, thus precluding its use in such an application.